The present invention pertains to cooling fans mounted to the shafts of electric motors and other similar dynamoelectric devices.
Many dynamoelectric devices, such as appliance motors for dishwashers, clothes washers, and the like, and large industrial motors, utilize a fan mounted on the rotating shaft of the device for cooling a stator, a rotor, a motor housing, and other components of the dynamoelectric device during operation. In one configuration, such a fan is mounted at one axial end of the motor and is configured to pull and/or push air through and/or adjacent the motor housing to cool the components. Such a fan can be mounted within a vented housing, as depicted in FIG. 1, to protect the rotating fan and to control the airflow into and through the fan.
As shown in the exemplary embodiment of FIG. 1, a motor is cylindrical in shape and a cooling fan is configured to fit within the radial footprint of the motor. The fan is configured to require a minimum amount of space, while providing sufficient air flow over the operating components of the motor. While axial flow fans may be used in some applications, it is often desirable to use radial flow fans that discharge air radially outwardly as the fan rotates. A fan grill and the motor housing are configured to direct this radial air flow across the critical components, such as axially of the motor, as illustrated by the air flow arrows moving from left to right in FIG. 1.
In order to control the direction of the air drawn into the fan, a typical straight blade fan will include a disc-shaped base or backing wall that blocks the flow of air axially through the fan. This feature allows the fan to generate a negative pressure at the center of the rotating fan facing the motor. This negative pressure in turn draws airflow from the opposite axial end of the motor, as represented by the airflow arrows at the right side of the motor housing shown in FIG. 1. This counterflow increases the heat dissipation between the solid body (the motor components) and the adjoining fluid (the airflow), thereby facilitating the cooling capability. This feature is due to an increase in the forced convection, which increases the fluid velocity and consequently increases the convection coefficient. In general, radial fans produce low airflow capacity and high head pressure, while axial fans produce high airflow capacity and low head pressure.
One type of radial fan is shown in FIG. 2. Details of this fan are found in U.S. Pat. No. 6,514,052, the disclosure of which is incorporated herein by reference. The fan includes straight, flat blades radiating radially outward from a central hub. The hub is mounted to the motor shaft for rotation of the fan as the motor is operating. The radial blades are flat and generally rectangular in shape.
Another motor and fan arrangement is illustrated in FIG. 3. In this configuration, the fan directs airflow over cooling fins projecting from the outside of the motor housing. The fan in FIG. 3 incorporates straight, flat blades radiating outward from a central hub which direct airflow radially outward across the base plate as the fan rotates with the motor.
One benefit of the straight blade radial fan designs shown in FIGS. 1-3 is that the fans may operate in opposite directions of rotation. In other words, the blades produce the same radial airflow whether the fan is rotated in the clockwise or counter-clockwise directions. This feature allows the fan to be mounted on either end of the motor shaft or to be used on a reversible motor without sacrificing any cooling capability. This attribute of the straight, flat blade fan provides a benefit over fans that utilize curved blades, such as axial flow devices, impeller devices, or uni-directional fans.
In order to meet more stringent design requirements, modifications in bi-directional fans (i.e., reversible fans) are continually sought to increase airflow capacity, increase fan/pump efficiency, increase the operating air pressure, and reduce the operating noise of the fan. As dynamoelectric device designs improve, the components operate at increasingly higher temperatures. These increased operating temperatures dictate the need for higher heat dissipation rates to maintain low temperature levels. In some cases, reducing the size of the dynamoelectric device dictates the need for increased air pressure to force air through smaller paths around the operating components. The cooling fan should meet these enhanced requirements without any increase in overall size, and sometimes with a decrease in size to match a decrease in size of the corresponding dynamoelectric device.
Moreover, noise reduction is often important, especially for dynamoelectric devices used in consumer appliances, such as dishwashers and clothes washers, as well as large industrial motors operating within specifications (e.g., operator health specifications). For example, noise levels above 85 dBA are undesirable in consumer appliances. Lower noise can provide a selling point for an appliance. Since the cooling fan can be the primary noise generator in these appliances, the focus for noise abatement is necessarily directed at the fan.